132
Baryogenesis
Further, the condition (4.225) is satisfied only for
mH ~ 10 GeV.
(4.231)
Thus, the more accurate calculation of the phase transition shows that the
current bound (4.195) is far from allowing a first-order phase transition, let alone
baryogenesis.
This analysis also indicates how the situation might be improved. The
strength of the phase transition, as measured by v(Tc)/Tc, can be increased by
substantially decreasing the effective (three-dimensional) scalar self-coupling 13,
as is suggested by (4.227). This requires a new. non-perturbative degree of
freedom and this happens when there are extra scalar degrees of freedom, as
occurs naturally in the supersymmetric version of electroweak theory to which
we now tum.
In summary, the standard model electroweak theory does not explain the
observed baryon asymmetry for two reasons: (i) because there is insufficient CPviolation; and (ii) because the phase transition is too weakly first order (in the
sense described earlier) to suppress the erasure of any baryon number asymmetry
produced in the symmetric phase.
4.11 Supersymmetric electroweak baryogenesis
We have already noted that the extra matter contained in the minimal
supersymmetric standard model (MSSM) might allow the alleviation of
the problematic features of the non-supersymmetric theory that preclude
baryogenesis at the level observed in nature. Supersymmetry entails the existence
of new fermions. In particular. there are charginos and neutraiinos, mass
eigenstates that are generically superpositions of the charged or neutral weak
gauginos and Higgsinos. There are also new bosons and we shall be specifically
concerned with top squarks. Diagonalization of the mass matrices of all of these
states generally leads to new sources of CP-violation. As we shall see, it is the
existence of new particles (and thereby of new sources of CP-violation), rather
than the supersymmetry itself, that might allow the MSSM to explain the observed
baryon asymmetry.
Now consider an expanding bubble of the broken phase, with the bubble
wall propagating through the hot plasma into the symmetric phase, perturbing
the (quasi-)particle distributions from eqUilibrium. Inside the bubble, baryonnumber non-conservation is small, because of exponential suppression by the
sphaleron's Boltzmann factor, provided that the sphaleron washout condition
(4.225) is satisfied. Effectively, baryon number is conserved inside the bubble.
However, outside the bubble. anomalous baryon-number non-conservation is
rapid. One way to see how baryogenesis occurs [60] is to think of the wall of
the expanding bubble feeling a 'wind' of particles in the symmetric phase. These
particles may pass through the wall into the broken phase or be reflected back
Baryogenesis
Further, the condition (4.225) is satisfied only for
mH ~ 10 GeV.
(4.231)
Thus, the more accurate calculation of the phase transition shows that the
current bound (4.195) is far from allowing a first-order phase transition, let alone
baryogenesis.
This analysis also indicates how the situation might be improved. The
strength of the phase transition, as measured by v(Tc)/Tc, can be increased by
substantially decreasing the effective (three-dimensional) scalar self-coupling 13,
as is suggested by (4.227). This requires a new. non-perturbative degree of
freedom and this happens when there are extra scalar degrees of freedom, as
occurs naturally in the supersymmetric version of electroweak theory to which
we now tum.
In summary, the standard model electroweak theory does not explain the
observed baryon asymmetry for two reasons: (i) because there is insufficient CPviolation; and (ii) because the phase transition is too weakly first order (in the
sense described earlier) to suppress the erasure of any baryon number asymmetry
produced in the symmetric phase.
4.11 Supersymmetric electroweak baryogenesis
We have already noted that the extra matter contained in the minimal
supersymmetric standard model (MSSM) might allow the alleviation of
the problematic features of the non-supersymmetric theory that preclude
baryogenesis at the level observed in nature. Supersymmetry entails the existence
of new fermions. In particular. there are charginos and neutraiinos, mass
eigenstates that are generically superpositions of the charged or neutral weak
gauginos and Higgsinos. There are also new bosons and we shall be specifically
concerned with top squarks. Diagonalization of the mass matrices of all of these
states generally leads to new sources of CP-violation. As we shall see, it is the
existence of new particles (and thereby of new sources of CP-violation), rather
than the supersymmetry itself, that might allow the MSSM to explain the observed
baryon asymmetry.
Now consider an expanding bubble of the broken phase, with the bubble
wall propagating through the hot plasma into the symmetric phase, perturbing
the (quasi-)particle distributions from eqUilibrium. Inside the bubble, baryonnumber non-conservation is small, because of exponential suppression by the
sphaleron's Boltzmann factor, provided that the sphaleron washout condition
(4.225) is satisfied. Effectively, baryon number is conserved inside the bubble.
However, outside the bubble. anomalous baryon-number non-conservation is
rapid. One way to see how baryogenesis occurs [60] is to think of the wall of
the expanding bubble feeling a 'wind' of particles in the symmetric phase. These
particles may pass through the wall into the broken phase or be reflected back